1. Field of the Invention
The invention relates to a method and device for improving the printing quality and the repetition accuracy of electrographical printers, in which a latent electric charge pattern of electrical signals is produced by means of an electrode matrix or the like, which opens and closes passages or apertures respectively between electrodes for exposing electrical fields for the attraction of pigment particles against an information carrier. Each electrode of the matrix is arranged on a carrier to completely surround one aperture of the matrix.
2. Description of the Prior Art
International patent application PCT/SE88/00653 discloses a method for developing pictures and text with pigment particles on an information carrier directly from computer generated signals, without the need for these signals to be intermediately stored for temporary conversion to light energy, which is necessary in photo conductive printers, e.g., laser printers. These problems have been solved by bringing the information carrier into electrical cooperation with at least a screen or preferably a lattice-shaped electrode matrix, which through control in accordance with the configuration of the desired pattern, the apertures of the matrix, which is galvanically connected to a voltage source, are at least partly opened and closed. An electric field is exposed through the open apertures for the attraction of the pigment particles against the information carrier.
This method, herein referred to as the EMS concept and as described in the above-mentioned patent application, may result in produced print which does not have high quality, particularly with repeated and continuous use.
The EMS concept refers to electrode matrices in which apertures or meshes in the matrix are defined and separated by simple electrodes, wherein the potential of every single electrode substantially influences the characteristics of the electric field on the pigment particles symmetrically in apertures adjoining the electrodes. This results in the attraction of pigment particles (herein called toner), not only in the mesh, which is surrounded by electrodes and the potential of which is intended to completely or partly open the mesh (herein called "black" voltage), but also in the exposed apertures of adjacent meshes. In electrode matrices with several mesh lines, meshes surrounded by simple electrodes will develop full-dots with intended extension and position, as well as half- and quarter-dots surrounding the full-dots. This results in an unsatisfactory edge definition and in certain cases a "blur" on the printed page. It is possible to change the potential of the adjacent electrodes, which are intended to close the apertures in the adjacent meshes (called "white" voltage) and reduce the problem of the undesired half- and quarter-dots, by skew setting the abovementioned symmetrical influence on the electrical field. This, however, leads to a potential difference between electrodes with "white" voltage and electrodes with "black" voltage (herein called contrast voltage), which in turn increases the manufacturing costs for the control electronics, as well as the electrode matrix.
The problems stated above are not limited to the EMS concept, but are also present wholly or partially in several electrographic printer concepts, where passage of toner is created in an electrical manner.
Common to all problems described here, another drawback of the known technique is that the printing quality, and thereby the readability, is influenced negatively resulting in reduced competitiveness and lower consumer value.